In prior art, continuous current control circuit modules of series string bulbs in order to maintain their electrical connection whenever any bulb assembly is broken or loosen have different structures. One is to provide an elastic metal slice in every bulb assembly being parallel connected to the bulb. If the bulb is broken, the metal slice becomes a shunt for connecting the series connected light string. Another design, in U.S. Pat. No. 6,084,357, discloses a string in which all of the bulb filaments in the set are individually provided with a non-avalanche shunt circuit to replace the elastic metal slice. The shunt circuit includes two or more silicon diodes. Since the diode is directional, it must be connected in correct direction. If there is any mistake in connection, the shunt will be out of work. Moreover, when the diode is used, the temperature will be increased rapidly to about centigrade 64 degree, that could cause the solved of the holder of the bulb and even leads a fire danger. There is an improvement by inventor of this application, which uses a triode to replace the known diode that overcomes the drawback in prior art.